One of the most
important discoveries of 1996 involved the first clear evidence for extrasolar
planets orbiting several sun-like stars. More than a dozen candidates for
planets around nearby stars have excited the imagination of astronomers and
nonprofessionals alike. If it can be demonstrated that planetary systems are a
common occurrence among the billions of stars in our galaxy, then the
possibility of extraterrestrial life may take on greater credibility. Indeed,
the idea that intelligent civilizations might exist on other planets could
become more compelling. Early reactions to extrasolar planet discoveries jumped
at the possibility that they might contain water and thus harbor life.1
However, a closer look at the accumulating data suggests that our solar system
may be unique in its life-supporting planetary arrangement.

Both the
possibility of life in other planetary systems and the apparent uniqueness of
our solar system have interesting religious implications. Even if the
possibility of extraterrestrial life were to increase, Christian thinking should
be able to accommodate such ideas with perhaps some theological adjustments
concerning the singular nature of the Incarnation. Roman Catholic theologian
Father Theodore Hesburgh states: "It is precisely because I believe
theologically that there is a being called God, and that he is infinite in
intelligence, freedom and power, that I cannot take it upon myself to limit what
he might have done... Finding others than ourselves would mean knowing him
better."2Protestant
astronomer Owen Gingerich of the Harvard-Smithsonian Center for Astrophysics
agrees: "In Genesis there's a sacred story being told that focuses on us.
But there is nothing that precludes intelligent life elsewhere in the
universe."3However, if the evidence continues
to point toward the uniqueness of our solar system, many current assumptions of
popular culture would be challenged.

For many scientific
devotees, extraterrestrial intelligence has become an article of faith,
following the postmodern trend to demote humans from any unique place of
significance in the universe. Carl Sagan,who believed that every sun-like star
would have planets,4was one of the most vocal
exponents of this view. The movie based on his novel Contact expresses
this faith that something out there is "greater than ourselves" and
"none of us are alone," repeatedly telling us, "if we are alone
in the universe, there sure is a lot of wasted space." However, growing
evidence suggests the possibility that the entire universe was necessary to
produce the conditions for intelligent life on a single planet.5

Extrasolar
Planet Background

The history of
extrasolar planetary searches suggests that some caution is needed in assessing
recent evidence. Early in the twentieth century, astrometric evidence from
Barnard's star, a nearby red dwarf one-seventh the mass of the Sun, indicated a
slight wobble which seemed to imply gravitational interaction by one or two
Jupiter-mass planets in decade-long orbits. However, by 1980 further work showed
that the wobble of Barnard's star was more likely the result of a companion star
too small to observe.6

Double-star systems
like Barnard's tend to rotate around each other in larger orbits than the tiny
wobble of a star with a planetary system. The mass of an unseen companion is
estimated from the amount of wobble detected from a visible star. Masses between
about forty and eighty Jupiter masses usually qualify as brown-dwarf stars,
defined as objects that form like other stars by gravitational collapse of a
dust cloud rather than from a stellar disk, but are too small to sustain the
nuclear fusion processes that energize most stars.

Planets condense from
materials in the disk produced by the collapse and rotation of a newly forming
star, and are believed to have masses less than about ten Jupiter masses. In
standard theories of planetary formation, matter in the protoplanetary disk
collides and coagulates into planetesimals ranging up to ten kilometers in size.
The planetesimals attract each other by gravity to trigger a sequence of mergers
that produces the inner rocky terrestrial planets, and the outer rock-and-ice
cores that seed the giant gas planets.

Because giant planets
require such a large amount of material, they should form only in regions
several astronomical units (AU = Earth-Sun distance) from their host star. Only
in these outer expanses of the disk (greater than about five AU) is it cool
enough for ice to form out of water molecules, roughly tripling the amount of
solid material available for planet making. When the ice-and-rock core reaches
about ten Earth masses, it begins to attract huge amounts of hydrogen and helium
gases and expands to about one Jupiter mass (318 Earth masses) until its gravity
can begin to tear a gap in the disk that feeds it, thus stopping its growth.
With this model, theorists were successful in accounting for the solar-system
sequence of inner rocky planets (Mercury to Mars) and outer gas planets (Jupiter
to Neptune) beyond five AU, but were completely surprised by the orbits of the
new Jupiter-size extrasolar planets.7

Planet discoveries
around sun-like stars began in 1995 and proliferated in 1996 with a new
generation of computers and optical instruments. Since planets are about a
billion times fainter than their host star, they are virtually impossible to see
by direct methods. An indirect method involves searching for a tiny wobble in
the motion of a star as it and any companions it may have orbit about their
common center of mass. Although the gravitational interaction between a star and
planet is too small to observe directly, the radial velocity (back and forth
along the line of sight) alternately increases and decreases the wavelength of
light from the star, causing an alternating Doppler shift toward the red and
blue end of its spectrum.

The amount of a star's
Doppler shift determines its velocity. The shift in the wavelength due to a
Jupiter-size planet is only one part in ten million. In the Doppler shift, the
periodic variation reveals the period of a planet's orbital motion. The velocity
of the star and the period of its motion can be analyzed to determine the radius
of the orbit (from Kepler's laws) and the minimum mass of the planet (from
Newton's laws), but the unknown inclination of its orbit allows for a larger
wobble than its apparent radial motion and, thus, a larger possible massˇup to
a factor of about two. The shape of the periodic variation curve reveals the
shape of the orbit. A circular orbit produces a perfect sine wave while an
eccentric orbit produces an irregular variation which can be analyzed by
computer to determine the orbital shape.

Extrasolar
Planet Evidence

Extrasolar planet
discoveries around sun-like stars have revealed two new and unexpected types of
planetary objects: hot-Jupiter planets with small circular orbits and
eccentric-Jupiter planets with elongated orbits. In October 1995, Swiss
astronomers, Michael Mayor and Didier Queloz, announced evidence of a companion
object orbiting the star 51 Pegasi about forty light years away. New computer
techniques revealed a periodic Doppler shifting of the light from the star,
suggesting a tiny wobble of up to eighty m/s caused by a planet of at least 0.46
of Jupiter's mass and a period of only 4.23 days in a circular orbit of just
0.05 AU radius. At this tiny distance from the sun, the 51-Pegasi planet has a
surface temperature of about 1800 deg. C. making it the first of several
"hot-Jupiter" planets.8

During 1996, Geoffrey
Marcy and Paul Butler of San Francisco State University announced the discovery
of six new Jupiter-size planets in a survey of 120 nearby sun-like stars over a
period of about ten years. Using a refined form of the method of Mayor and
Queloz, they achieved a three-fold improvement in accuracy, detecting radial
motions to about three m/s. Since Jupiter, which contains more mass than all the
other planets combined, causes the Sun to move at speeds of up to 12.5 m/s,
Jupiter-size planets can be readily detected. With these accuracies, however,
Earth-size planets cannot be detected. Even Jupiter-size planets with periods of
several years require that data be collected over a long enough time to
determine their orbital periods.9

Three of Marcy and
Butler's new planets were hot Jupiters with nearly circular orbits at distances
of only 0.11 AU or less from their host stars (55 Cancri, Tau Bootis and Upsilon
Andromedae), having periods less than fifteen days and minimum masses ranging
from 0.68 to 3.87 Jupiters. Another planet discovered in 1997 around the star,
Rho Corona Borealis, has a minimum mass of 1.1 Jupiters and a circular orbit at
a distance of 0.23 AU and period of 39.6 days. Since it is much closer than
Mercury to the Sun, it also appears to qualify as a hot Jupiter. These
discoveries showed that the 51-Pegasi planet was not as unusual as it first
seemed.

Marcy and Butler also
found an eccentric-Jupiter planet around the star 70 Virginis with a 117-day
elongated orbit ranging from 0.27 to 0.59 AU and a mass of at least 6.5 Jupiters.
This discovery led David Latham at Harvard to suggest that an object found in
1988 with a mass of at least nine Jupiters orbiting the star HD 114762 in an
84-day elongated orbit that varies from 0.22 to 0.46 AU was also an
eccentric-Jupiter planet rather than a small brown dwarf as first assumed. A
third eccentric Jupiter was discovered in 1997 orbiting the star 16 Cygni B in a
triple star system. It has a mass of at least 1.5 Jupiters and a 2.2 year orbit
varying widely between 0.6 and 2.8 AU.

A few of the recent
extrasolar planet discoveries appear to be a little more like Jupiter, but still
rather puzzling. Two of Marcy and Butler's first six planets included a second
one around 55 Cancri, having a period of about twenty years, an orbital radius
of about five AU, and a mass of at least five Jupiters. Another one around 47
Ursae Majoris has a minimum mass of 2.3 Jupiters, a period of 3.0 years, and a
nearly circular orbit at a distance of 2.1 AU, still less than the expected
distance for a giant planet. At this distance it would have a surface
temperature of about 85 Deg. C. low enough to allow for liquid water but with a
huge surface gravity that would be problematic for life.

Two other Jupiter-like
planets, both orbiting the star Lalande 21185, were announced in 1996 by George
Gatewood of the University of Pittsburgh.10
Analyzing data from fifty years of photographic observations and eight years of
photoelectric measurements, he detected one planet with about 0.9 of a Jupiter
mass at 2.2 AU and a period of about 5.8 years, and another with about 1.1
Jupiter masses at eleven AU and a period of about thirty years.

An objection to
extrasolar-planet interpretation of the Doppler evidence has been raised by
David F. Gray, who claims that the perceived periodic motions of some stars may
be the result of their pulsations rather than planetary interactions. Gray
claims that oscillations in the star's atmosphere could reshape spectral lines
by Doppler shifting and gives evidence that the spectral lines for 51 Pegasi
vary in shape with a 4.23 day period. This objection has been countered by
Mayor, Queloz, Marcy and Butler by pointing out that pulsations should change
the brightness of 51 Pegasi, but it has a constant brightness to one part in
five thousand. They also point out that only one period has been detected, with
none of the other overtones or oscillation modes that should accompany
pulsations.11

Extrasolar
Planet Implications

The discovery of
extrasolar planets around sun-like stars may at first seem to offer new hope for
the existence of planetary systems like ours that would support extraterrestrial
life. But the unexpected nature of these planets has raised new doubts about the
possibility that any of them might harbor life. Hot-Jupiter planets and
eccentric-Jupiter planets have initiated a new generation of theories about
planetary formation and the uniqueness of our solar system. Evidence so far
seems to indicate that our solar system is highly unusual in its life-supporting
planetary arrangement.

The strangest of the
new planets are the hot Jupiters with minimum masses ranging from 0.46 to 3.87
Jupiter masses and orbital radii less than 0.23 AU. They all are most likely gas
planets with surface temperatures well above the boiling point of water. Revised
theories suggest that they might have formed beyond five AU from their host
stars in a dense protoplanetary disk, which then slowed them down and caused
them to spiral inward. Such a process would obliterate any small, inner
terrestrial planets congenial to life as we know it on Earth.12

The eccentric Jupiters
have longer periods (84 days to 2.2 years) and larger orbits, but with huge
eccentricities (0.35 to 0.67) and larger distances from their host stars out to
as much as 2.8 AU. New theories suggest that two or more super-Jupiters forming
from a dense protoplanetary disk might then interact with each other
gravitationally, causing some to be thrown into eccentric orbits or even tossed
free of the star (as seen in May 1998 in the first photo of a possible
extrasolar planet). Such eccentric giants would gravitationally disturb and
eventually collide with smaller inner planets, again precluding life-supporting
planets like Earth.

The Jupiter-like
planets are a little more like those in our solar system but still diverge from
ideal conditions for life. Even though they have nearly circular orbits and are
further from their host stars than the hot Jupiters, their huge mass (0.9 to 5
Jupiter masses) suggests that they are lifeless gas planets like Jupiter with
stormy, violent winds and intense gravity. Those that are near the habitable
zone (about two AU) would also tend to upset the orbital stability of any
smaller Earth-like planets. In cases where systems of two planets have been
identified (55 Cancri and Lalande 21185), the outer ones are most like Jupiter,
but each has an inner hot Jupiter that would again preclude terrestrial planets.

Although current
methods can only detect Jupiter-size planets, the orbits detected so far appear
to reduce the possibility of smaller life-supporting planets. Most of the 120
stars surveyed by Marcy and Butler do not appear to have Jupiter-like planets at
all (in either size or period), but they could have smaller undetected planets.
However, such Jupiter-like planets may be necessary for the development of
complex life forms on smaller planets. Earth is struck by asteroids large enough
to cause mass extinctions of species about once every fifty to a hundred million
years. Computer simulations by George Wetherill show that without Jupiter in its
present stable orbit beyond Earth, sweeping up most killer asteroids and comets
(as seen with comet Shoemaker-Levy 9 in July 1994), this Earth-collision rate
would be about a thousand times greater, too large to permit the development of
higher forms of life, if any at all.13Thus none of the 120 sun-like stars surveyed so far appear to offer much
hope for life, greatly decreasing the probability factors that Sagan and others
have presumed as the basis for the existence of extraterrestrial civilizations.

Religious Responses

The possibility
of extraterrestrial civilizations has become almost an article of faith for many
contemporary scientists, despite the lack of any evidence for their existence,
and the discovery of extrasolar planets has not added much hope. Just as ancient
civilizations looked to the skies for their deities, many modern materialists
hope for radio signals from space to confirm their faith in higher intelligences
on extrasolar planets. Such blind enthusiasm offers a naturalistic substitute
for faith in God, and can easily lead to alternative religious innovations. Such
influence is evident in three, nineteenth-century religious movements that
incorporated extraterrestrial life into their religious thought: the New
Jerusalem or Swedenborgian Church, the Mormon Church, and the Seventh-Day
Adventist Church.

The Swedish scientist
and sage Emmanuel Swedenborg (1688˝1772) claimed to have had conversations
with extraterrestrials and worked out a theology that included them. The
Swedenborgian Church was organized in London (1787) and now has about 40,000
members. Besides the Book of Mormon, Joseph Smith (1805˝44) provided
his Church of Jesus Christ of Latter-Day Saints with other Scriptures, including
Doctrine and Covenants and The Pearl of Great Price, which taught
that there are many inhabited worlds in the universe. This doctrine is given
considerable emphasis in the theology of the Mormons, who now number some eight
million members. Beginning in 1846, Ellen G. White (1827˝1915) began having
visions involving extraterrestrials. When she and her associates founded the
Seventh-Day Adventist Church in 1863, White developed a theology involving
extraterrestrials in which sin occurred only on Earth. In The Story of
Patriarchs and Prophets, she taught that Christ passed from star to star to
superintend the sinless intelligences of other worlds. This cosmic conception of
Christianity has spread to a worldwide membership of about 4.4 million.14

Most scientific
interest in extraterrestrial intelligence today is based on naturalistic
arguments concerning the probability of life arising on extrasolar planets, but
it is often closely associated with religious themes, such as a yearning for
meaning, wisdom, and even immortality which is presumably possessed by
extraterrestrial higher intelligences.15 Although
plausible arguments are used to further this faith, no evidence has yet been
found to support it. If the evidence for the lack of Earth-like extrasolar
planets that can support intelligent life continues to accumulate, the only
saving hope for many naturalists will fail. It is too early to say for sure, but
the most important lesson that might emerge from such evidence is the uniqueness
of our solar system with its life-sustaining planetary arrangement as a special
gift from God to his creatures on Earth.

9A
good summary of recent discoveries is by Robert Naeye, "The Strange New
Planetary Zoo," Astronomy (April, 1997): 42˝9. Marcy and Butler
maintain a good website for current information on extrasolar planet discoveries
at http://cannon.sfsu.edu/~williams/planetsearch/planetsearch.html.